The well-known deficiencies in the surface properties of GaAs have stimulated many attempts to passivate the surface, reduce the surface state density, and unpin the surface Fermi level. There are many reasons why it is desirable to reduce the surface state density of III-V compound semiconductors and GaAs in particular. A reduction in surface state density would lead to improved field effect transistor (FETs) by lowering surface-related leakage currents and by increasing the Schottky barrier height. This would allow the fabrication of digital logic circuits with increased noise margins and relaxed requirements on threshold voltage uniformities of the component FETs. In addition, unpinning the surface Fermi level would allow advances in metal-insulator-semiconductor (MIS) capacitor and MISFET technology. For photonic devices, it is important for enhancing power output to reduce the surface recombination velocity and increase carrier lifetime. Passivation may be carried out prior to processing as a cleaning step, or after processing to remove process-induced damage.
A variety of wet chemical and photochemical treatments have been used to passivate GaAs but controversy remains as to their effectiveness in unpinning the Fermi level. Nonetheless, it is clear that photoluminesence (PL) yield is enhanced by using these surface treatments. In fact, passivation is operationally defined in terms of this enhancement in quantum yield. Although large increases in PL yield have been observed, the enhancements tend to be short-lived. We are aware of only one case where the enhancement persisted for more than a few hours (See H. H. Lee, R. J. Racicot, and S. H. Lee, Appl. Phys. Lett. 54,724 (1989). Occasionally, realtime PL monitoring has been used to monitor the effectiveness and kinetics of these wet chemical treatments.
To improve process control and management of environmental hazards, it is desirable to replace wet chemical processing with dry processing. In particular, plasma methods are effective for not only etching but also deposition, cleaning, and passivation. Recently, H.sub.2 plasmas operated at low pressure have been used to clean GaAs of residual As and As.sub.2 O.sub.3. Similar plasma treatments have also been reported for InP surface cleaning. A variety of reactor configurations have been employed: electron cyclrotron resonance (ecr) microwave, multi-polar, and rf diode. However, in these reports choice of optimal exposure times, which have ranged from 5 s to 30 min, and operating pressures, which have ranged from 10.sup.-4 to 10.sup.-2 Torr, is not discussed. Similarly, the consequences of reactor geometry or excitation mode choice have not been addressed. In one case the properties of Schottky diodes made on hydrogen plasma treated n-GaAs surfaces was studied as a function of the treatment temperature. The conclusion was that plasma passivation is ineffective for temperatures below 200.degree. C. (See A. Paccagnella, Appl. Phys. Lett. Vol. 55, No. 3, Jul. 17, 1989, pages 259-261). A similar conclusion has been made for hydrogen plasma treatment of InP surfaces. However, as we describe here, such conclusions depend on other plasma parameters. It is the recognition of the role of those parameters that leads to the inventive process.
Real-time monitoring of PL from GaAs during hydrogen plasma passivation clearly provides the means to investigate the effects of processing conditions such as exposure time, pressure, and temperature. (See U.S. patent application Ser. No. 07/402,030 filed Sep. 1, 1989). In addition, the kinetics of plasma-surface reactions can be monitored by measuring real-time changes in PL yield. In general, PL yield is sensitive to changes in surface recombination velocity (S) and band bending. Clearly, PL yield increases as S decreases. In addition, if the bands are bent and a space-charge field exists near the surface, separation of electrons and holes can lead to a reduction in radiative recombination efficiency. Thus, a reduction in band bending can increase PL yield. However, if S is reduced substantially, PL yield can increase despite an increase in band-bending. In n-GaAs, for example, passivation treatments tend to remove excess As from the surface and thereby reduce the As antisite defect density that pins the Fermi level near mid-gap. The Fermi level shifts to lower energy because the density of Ga antisite defects remains constant or even increases. Thus, the band bending increases. Ordinarily this would cause a decrease in PL yield as carriers are separated by the space-charge field. However, reduction of the near-mid-gap As antisite defect density decreases the non-radiative recombination rate causing an increase in PL yield.